This invention relates to traction control systems for AC locomotives and, more particularly to a torque maximizer, and a method and a system which maximizes torque and minimizes torsional vibration on a per axle basis.
In a modern conventional diesel-electric locomotive, a thermal prime mover (typically a turbo charged diesel engine) is used to drive an electrical transmission comprising a synchronous generator that supplies electric current to a plurality of electric traction motors whose rotors are coupled through speed-reducing gearing to the respective axle-wheel sets of the locomotive. The generator typically comprises a main 3-phase traction alternator, the rotor of which is mechanically coupled to the output shaft of the engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase armature windings on the stator of the alternator. These voltages are rectified and applied via a DC link to one or more inverters where the DC voltage is inverted to AC and applied to AC traction motors.
In normal motoring operation, the propulsion system of a diesel-electric locomotive is so controlled as to establish a balanced steady-state condition wherein the engine-driven alternator produces, for each discrete position of a throttle handle, a substantially constant, optimum amount of electrical power for the traction motors. In practice, suitable means are provided for overriding normal operation of the propulsion controls and reducing engine load in response to certain abnormal conditions, such as loss of wheel adhesion or a load exceeding the power capability of the engine at whatever engine speed the throttle is commanding or a fault condition such as a ground fault in the electrical propulsion system.
As is generally known, the 3-phase synchronous alternator in a locomotive propulsion system develops an output voltage which is a function of its rotor shaft RPM and the DC voltage and current applied to its field windings. The 3-phase output is converted to DC power by a 3-phase full-wave bridge rectifier connected to the alternator output windings.
The DC power is coupled to a DC link and supplied to a plurality of parallel connected inverters. Each inverter comprises a plurality of electronically controllable switching devices, such as gate turn-off thyristors (GTO's), which can be gated in and out of conduction in a conventional manner so as to generate an AC output for powering AC electric traction motors coupled in driving relationship to respective axle-wheel sets of the locomotive.
One factor affecting traction performance is the creep level of the locomotive's traction control subsystem. Accordingly, it is desirable to separately control the allowable creep level of each individual axle to maximize traction performance. Additionally, it is desirable to maximize the control system response whose function is to increase or decrease the allowable creep level.
Another factor affecting traction performance is the level of torsional resonant vibration in the mechanical drive train, which is comprise of a locomotive axle and its associated two wheels, the motor to axle gearbox, the induction motor, and the induction motor drive. In particular, during operation in certain regions of the adhesion characteristic curve, the mechanical drive train may experience a net negative damping which produces severe vibration levels at the system's natural frequencies. As is well-known, an adhesion characteristic curve graphically represents the coefficient of friction versus percentage creep. At zero percent creep, maximum damping on the mechanical system is represented. As the percent creep level increases in the portion of the characteristic curve to the left of its peak, the damping effect on the mechanical system decreases to a value of zero at the peak. For increasing percent creep values to the right of the peak, the damping provided to the mechanical system becomes a larger negative number.
The natural frequencies of a system are a function of the drive train component materials and geometries which vary slightly over the life of a locomotive due to wear and tear. Dependent upon the magnitude and duration of the vibration periods, the drive train may be damaged. Accordingly, it is desirable to minimize torsional resonant vibration in order to maximize traction performance.
U.S. Pat. No. 5,841,254 discloses a control system in which creep level and torsional vibration level are utilized to maximize traction performance. This control system is useful for a wide variety of applications and overcomes problems known to those skilled in the art.